Orange and T-Mobile will offer UK customers the ability to roam across each other's networks from October.
The mobile network carriers announced last week that, when a user loses signal in the UK, their phone will switch to use the other company's network.
The agreement is a part of their joint venture 'Everything Everywhere', which started on 1 July at Hatfield, Hertfordshire, news agency Reuters has reported.
The roaming service, which will be launched on 5 October, will allow Orange users to switch to T-Mobile and vice versa whenever needed. However, users will have to opt-in for the facility.
According to the report, opt-in is required to deal with certain handset issues that may affect the phone while entering another's network.
CEO of Everything Everywhere Tom Alexander has said that, over time, roaming will become fully automatic, allowing users to select any approved network available.
The joint venture, Everything Everywhere is now the UK's largest mobile provider with more than 30 million customers, ahead of rivals Vodafone and O2.